The invention relates to a synchronous motor, in particular an encoderless synchronous motor. Furthermore, the invention relates to an encoderless motor system with such a synchronous motor, and a method for operating the encoderless motor system.
A synchronous motor normally has a number of stator coils which are arranged in the vicinity of an armature with at least one permanent magnet in order to drive the armature. The armature is normally designed in the form of a rotor, the permanent magnet being arranged on the rotor so that it produces an armature magnetic field in a radial direction with respect to a rotor axis of the rotor. The armature of the synchronous motor is driven by applying an appropriate stator current to each of the stator coils with the help of appropriate control signals, thus giving rise to a resulting drive magnetic field. The stator currents are controlled in such a way that the drive magnetic field runs essentially perpendicular to the direction of the armature magnetic field produced by the permanent magnet in order to produce a greatest possible torque. In order to always apply the stator currents so that the direction of the magnetic field produced by the stator coils runs perpendicular to the armature magnetic field, the position of the armature or rotor position must be known. To operate the synchronous motor, the position of the armature must therefore be continuously determined or estimated so that the stator coils can be optimally controlled depending on the determined position of the armature. Position detectors are normally provided to determine the position of the armature or rotor position.
In the case of an encoderless synchronous motor, the position of the rotor is estimated with the help of an anisotropy of a resulting inductance in the stator coils of the stator, i.e. when the synchronous motor is operating, different resulting inductances can be measured in the stator coils depending on the rotor position, by means of which the position of the armature or rotor can be estimated. To do this, measuring signals are superimposed on the control signal for applying the stator currents for the stator coils so that, in addition to the drive magnetic field, an alternating magnetic field is produced, wherein the current flows through the stator coils produced by the measuring signals depend on the resulting rotor-position-dependent inductance of the synchronous motor. Because, as previously described, the resulting inductance of the synchronous motor depends on the position of the rotor, the rotor position can be estimated from the measuring currents which are produced by the measuring signals.
In order to determine the rotor position as accurately as possible, it is necessary that the characteristic of the resulting inductance of the synchronous motor changes as much as possible depending on the rotor position, i.e. it is anisotropic, the difference of the resulting inductance between one direction of the armature magnetic field and a different direction thereto being as great as possible. However, with conventional rotors, the low anisotropy of the resulting inductance leads to the rotor position only being estimated inaccurately, so that the drive magnetic field does not run exactly perpendicular to the armature magnetic field. As a result, the synchronous motor cannot be operated with the optimum torque.